ROSAT SCIENCE

The following text has been taken from the science section of the 1996 Senior
Review proposal for the US ROSAT Science Data Center (Dr. Rob Petre, PI).
It was compiled by members of the ROSAT User Committee and USRSDC staff.

Subject Index

When used to observe the nearest celestial objects, those within the solar
system, the ability of the HRI to perform high spatial resolution observations
of low surface brightness objects has resulted in some of ROSAT's most
dramatic discoveries. The HRI has detected two extraterrestrial solar system
objects, and both have yielded surprising, even revolutionary, results. More
importantly, ROSAT has proven the value of X-ray observations of our own back
yard.

Well before the launch of ROSAT, it was known that X-rays are produced by
Jupiter, predominantly from the poles (Metzger et al. 1983, JGR, 88, 7731).
ROSAT HRI data have allowed a much better characterization of the spatial
and temporal variations of this emission, and have shown that the variability
is largely consistent with the much more completely studied UV emission from
the aurora, with a strong north-south asymmetry and a brightness variation
with longitude. In addition, ROSAT HRI observations have revealed X-ray
emission from elsewhere in the Jovian system: they reveal a belt of
equatorial X-ray emission, and provide indications of X-ray emission from the
foot of the Io flux tube (IFT). The responsible mechanisms are not known
(Waite et al. 1994, JGR, 99, 14788; 1995, JGR, 100; 1995, Science, 268, 1598).

The most dramatic observations of Jupiter occurred during the comet
Schoemaker-Levy 9 impacts. A brightening of the X-ray emission at the
north pole was associated with the impact of two comet fragments (near the
south pole). Waite et al. (1995) have concluded that these represented
impact-induced brightenings of the aurora. The process responsible for the
brightenings is still a mystery: the brightenings did not occur at the at
the conjugate points of the impact sites, and thus a more complicated process
is required than precipitation of particles energized by the impact and
carried along magnetic field lines.

Fundamental questions associated with the Jovian X-ray emission -- the
identity and energy distribution of the particles that excite the emission,
the process responsible for the equatorial emission, the possible correlation
between the emission and the IFT -- can be answered largely by continued
ROSAT observations of spatial and intensity variability, but require an
extensive campaign of monitoring, in conjunction with observations in other
wavebands.

The most exciting surprise to arise from ROSAT observations in the past year
was the detection of strong, extended, and highly variable X-ray emission
from Comet Hyakutake as it passed within 0.1 AU of the Earth (Lisse et al.
1996, IAUC 6373; Science, submitted). Prior to the observation, the most
optimistic prediction of cometary X-ray flux, based on the published
literature (e.g., Ibadov 1990, Icarus, 86, 283), translated to an expected
HRI rate of ~0.01 s^-1. The detection of the comet during a real time pass,
at an average count rate of ~4 s^-1, was greeted with astonishment. The
count rate was observed to vary by more than a factor of five on a timescale
of hours. The X-ray emission arises from a nearly hemispherical shell,
symmetrical about the comet-sun direction, with a diameter of ~20,000 km.
No X-ray flux is observed from the nucleus. Lisse et al. (1996) discuss a
number of possible emission mechanisms, including resonant scattering of
solar X-rays, collisions between cometary and interplanetary dust, and
interaction between the comet and the solar wind, and show that none provides
a satisfactory explanation. Follow up HRI observations are planned for late
June, 1996.

Confirmation that Hyakutake is by no means unique comes from a subsequent
search of the ROSAT All-Sky Survey database, which thus far has yielded
detection of three comets (Dennerl et al. 1996, IAUC 6404, 6413), at least
one of which has extended emission. The spectral information from the PSPC
remove any ambiguity about the nature of the emission (i.e., X-rays vs. UV
leakage in the HRI), and suggest a thermal origin (kT ~ 0.4+/-0.1 keV)
indicative of shock (solar wind) heated gas, plus a possible oxygen
fluorescence line.

Cometary X-ray observations are clearly in their infancy. How the X-ray
emission scales with other cometary properties is yet to be determined, as
is what X-rays can reveal about the nature of comets and the solar wind.
Additional observations of comets with a range of luminosities, heliocentric
distances, masses, and mass loss rates will begin to answer these questions.
The key fact is that a new area of X-ray astronomical study has been opened,
and will undoubtedly attract to ROSAT, ASCA, AXAF, an entirely new group of
astronomers.

Stellar observations continue to represent one of the largest portions of
ROSAT observing time (~25 percent). There are a number of types of stellar
observations for which the ROSAT HRI offers unique capability. These include:

-
Stellar clusters/associations. Star clusters and associations are the
Rosetta Stone of stellar astronomy. Each provides a coeval sample of stars
with identical chemical compositions. Wide field X-ray images permit us to
map the X-ray emission from these clusters. Observations of a single cluster
permit one to determine the dependence of coronal activity on mass and
rotation; observations of a range of clusters permit the most accurate
determinations of the time-decay of activity, and the dependence of activity
on chemical composition and other parameters. X-ray observations also permit
one to identify likely members of sparse clusters based on activity levels.
The large field of view permits one to observe a significant fraction of
a nearby clusters in only a few pointings. Only half of the 48 known open
clusters have so far been observed with ROSAT, and most of those are fairly
young. Over the next few years, observations will likely concentrate
on middle-aged clusters. Such observations are necessary to determine why
the
Hyades and Praesepe, with similar ages, have such different X-ray
luminosity functions.

- Compact regions The youngest clusters/associations tend to be spatially
compact, and source confusion is a serious problem, even at the ~1 arcmin
resolution of the PSPC or ASCA/SIS. Recent HRI observations of the
Orion nebula (Gagne et al 1995, ApJ, 445, 280; Caillault et al. 1994, ApJ,
432, 386), the
Rho Oph complex (Montmerle et al., in preparation), and the R
Corona Australis cloud (Walter et al., in preparation) have shown that many
PSPC sources are resolved into groups of 2 or more point sources, each with
stellar counterparts.

- Embedded Protostars The ROSAT HRI is surprisingly effective in penetrating
high column densities to observe embedded and obscured X-ray emitters. This
is most evident in studies of young stellar objects in nearby star formation
regions. PSPC observations of the core of the r Ophiuchi cloud, for example,
revealed several dozen faint X-ray sources from Class II and III infrared
sources, which are associated with classical and weak-lined T Tauri stars
(Casanova et al. 1995, ApJ, 439, 752). This is not a new result, as many
unembedded CTT and WTT stars are X-ray emitters. However, they reported that
up to 5 Class I infrared sources, associated with the later stages of
protostellar accretion, may be been seen. However, the fields are crowded
and alternative Class II/III identifications were possible. A 40 ksec follow
up with the HRI in 1995 clearly shows that one Class I source, IRS 43, is
coincident with an X-ray source with arc second accuracy (Boresight is aligned
to visible stars). In addition, the X-ray emission showed a powerful flare
on hour timescales, similar to flares seen on CTT/WTT stars. The obscuration
to this star is estimated from the infrared spectrum (it is optically
invisible) to be Av ~ 44, equivalent to a column density NH ~ 2x10^23 cm^-2.
This observation, together with a similar discovery of flaring X-rays from
a protostar made with ASCA (Koyama et al. 1996, PASJ submitted), establishes
that low mass protostars are X-ray emitters.

This discovery may have great importance for our understanding of star
formation and protostars. First, protostellar X-rays should photoionize the
immediate environments, including the edges of the cold circumstellar disk.
This may provide the magnetic coupling essential in magneto-centrifugal
acceleration models of bipolar flow ejection. X-rays may thus play a central
role in the flows produced by all protostars. Second, protostellar X-rays
may photoionize ambient material, slowing ambipolar diffusion and thereby
reducing infall. X-rays might thus be involved stopping the star formation
process and thus affecting the initial mass function. Third, the X-ray flares
should be similar to solar flares and be accompanied by ejection of energetic
protons. There is considerable evidence from meteoritic studies that the
solar nebula was bombarded by energetic particles at levels much higher than
produced by the contemporary Sun. These particles might conceivably also be
responsible for isotopic anomalies seen in ancient meteorites. Altogether,
the HRI is providing a new window onto unexpected energetic phenomena in the
star formation process.

- Visual binaries (3-30 arcsec separations), which the HRI can resolve, show
that the primaries and secondaries of pre-main sequence pairs seem to have
different X-ray luminosity functions, even after accounting for the mass
differences (Walter & Liu, in preparation). The cause is unknown, but raises
questions about the completeness of X-ray selected samples.

- Temporal variability The HRI is quite useful for tracking the variability
of sources, both on short timescales (because there is no wire mesh support
structure) and on long timescales (because of the instrumental stability).
Ongoing programs include measurements of a stellar cycle on EK Dra (Guinan
1996, in preparation) and
AR
Lacertae, and daily observations of Sigma Ori (Schmitt et al. 1996, in
preparation) and the
Orion
Nebula (Caillault et al.
1996, in preparation) over timescales of a month to study rotational
modulation and flaring.

One of the more dramatic results concerns the O7 V star Theta 1 Ori C, the
brightest star in the Orion Trapezium. Caillault et al. (1996) discovered
a clear periodicity with an amplitude modulation of 35 percent and a period
of 15-16 days. The X-ray maxima line up exactly with the maxima from Ha
data, which show a period of 15.42 days. This is the first example of large,
periodic X-ray variability on a single O or B-type star. A survey of
archival IUE data suggests the period and phase have persisted for nearly a
decade (Walborn & Nichols 1996, in preparation). The only model that might
explain the observation is completely at odds with current thinking about O
stars: that Theta 1 Ori C possesses a large magnetosphere whereby the X-ray
variability is caused by rotational modulation of a magnetic active region.

ROSAT PSPC observations in 1992 and 1993 showed that the peculiar star
Eta
Carinae (probably the most massive Galactic star), a known X-ray source,
was in fact a variable X-ray source as well. This discovery, as surprising
as it was, was confirmed in even more dramatic fashion using
HRI
data. A
comparison between HRI images taken in 1992 and 1994 revealed significant
structural differences. In particular, the 1994 HRI image clearly showed
a strong core of emission at the position of Eta Car, which was absent in the
1992 image. Subsequently it was realized that the minimum of the X-ray
emission was simultaneous with a minimum seen in the He I 10830A line
strength, suggesting that the X-ray emission might be periodic at the 5.52
year period of the He I line. If so, then the X-ray flux should vary
continuously and reach the next minimum in 1998. More recent observations
with ASCA and XTE have indeed suggested that the X-ray emission from Eta Car
continues to vary. But in order to constrain models of the X-ray emission
we need to have some knowledge of the size of the emitting region during the
crucial interval before the predicted minimum in 1998. Until the launch of
AXAF, the ROSAT HRI is the only instrument capable of providing this
fundamental information about the extent of the X-ray core.

ROSAT HRI's unique combination of high spatial resolution and sensitivity is
providing critical new insight into the results of stellar interactions in
globular cluster cores. It has already more than tripled the number of X-ray
sources known in clusters, while simultaneously providing the positional
accuracy needed to identify their optical counterparts. Pointed observations
of 43 Galactic globular clusters have been made or are in progress with ROSAT,
35 with the HRI. These studies have shown that:

- Low-luminosity X-ray sources are indeed far more numerous in clusters than
their high-luminosity counterparts. Thirty low-luminosity sources are now
known in 18 clusters (Verbunt 1996, in ASP Conf. Series 90, eds. Milone &
Mermilliod, p. 163), which more than triples the number identified with
Einstein.

- In nearly every globular in which one or more low-luminosity sources have
been detected, sources have been detected at or near the detection limit of
the observation, indicative of a rising luminosity function. Estimates of a
power-law slope of the luminosity function from current data are in the range
a alpha=0.5-1.0 (Grindlay et al. 1995, ApJ, 455, L47; Johnston & Verbunt 1996,
A&A, preprint).

- Four of the eight clusters for which the deepest exposures have been made
contain three or more faint sources (Cool et al. 1995, ApJ, 439, 695; Grindlay
1993, in ASP Conf. Series 50, eds. Djorgovski & Meylan, p.285; Hasinger et al.
1994, A&A, 136, 331). In every case, the spatial resolution of the HRI has
been essential to separate the sources from one another in order to properly
assess their numbers, luminosities, and spatial distribution in the cluster.

- Low-luminosity sources are present in clusters with a wide range of
properties. The only cluster for which an observation sensitive to sources
as faint as Lx<10^32 erg s^-1 detected no sources in the cluster is M71, which
has both a low central density and a low mass.

- Some of the dim sources have highly variable X-ray luminosities (e.g.,
Hasinger et al. 1994), so that observations at different epochs detect a
different (though generally overlapping) subset of the cluster sources. An
accurate assessment of the number of sources in a cluster can thus require
repeated observations.

At the same time, by providing X-ray source positions accurate to a few arc
seconds, the ROSAT HRI has made it feasible to identify optical counterparts
even in crowded cluster cores. Promising identifications have now been made
for sources near the cores of three different clusters (Cool et al. 1995;
Edmonds et al. 1996, preprint; Bailyn et al. 1996, ApJ, submitted), and
follow-up spectroscopy of three stars in one cluster has revealed the emission
lines characteristic of accretion disks (Grindlay et al. 1995). Studies by
several groups are now underway to further characterize these once elusive
binary stars through their optical properties.

The fraction of Galactic globular clusters observed to date is small,
particularly observations sensitive enough to readily detect the largest class
of sources. Further understanding of the nature and origin of these compact
binary stars and their role in cluster dynamics will profit from systematic
surveys of large, unbiased samples of clusters. A primary goal of studies of
X-ray sources in globulars is to determine the conditions necessary for their
formation, and to understand their evolutionary relationship to populations
of other cluster stars such as millisecond pulsars, primordial binary stars,
and various possible merger products in cluster cores. Intercomparisons
of the relative numbers of low-luminosity X-ray sources vs. millisecond
pulsars, blue stragglers and extreme blue horizontal branch stars will
provide valuable constraint s on ongoing efforts to understand the
consequences of stellar interactions in cluster cores. On the optical side,
HST is providing greatly improved information about the distribution of
unusual blue stars in a large number of clusters. Only the ROSAT HRI can
provide the complementary database of low-luminosity X-ray sources in
clusters. By assessing the numbers, luminosities, variability
characteristics, and radial distribution of dim sources in clusters with
a wide variety of properties, a more complete picture of cluster evolution
can be developed.

Cataclysmic Variables (CVs): Our present and rapidly evolving view of
Cataclysmic Variables (CVs) is based in large part upon the contributions
made by ROSAT during the all-sky and pointed phases of the mission. Of
particular note is the explanation of the period gap, a deficit of magnetic
accreting systems typified by AM Hercules with orbital periods in the range
of two to three hours. The doubling by ROSAT of the AM Her type systems has
essentially shown that the gap was purely a selection effect (Beuermann &
Schwope 1993; Proc. ASP Meeting, San Diego; Watson 1993, Adv. Sp. Res. 13,
125). Other discoveries of ROSAT related to magnetic CVs such as very long
period systems (Shafter 1994, Proc. 1994 Padua CV meeting; Beuermann 1994,
Proc. 1994 Padua CV meeting), very short period systems close to the
theoretical minimum (Osborne et al. 1994, MNRAS, 270, 650), and transitional
systems (Mason et al. 1992, MNRAS 258,749) provide unique laboratories to test
future theoretical models.

The contribution of ROSAT to the advancement of our understanding of
non-magnetic systems has also been profound. In addition to the detection of
recent classical novae (Lloyd et al. 1992, Nature, 356, 222; Shore et al. 1994,
ApJ, 421, 344; Ogelman et al. 1993, Nature, 361, 331) and evidence for the
existence of an extended component possibly related to an accretion disk
corona (Wood, J. H. 1992, PASP, 104, 780; see Beuermann, K. & Thomas, H.-C.
1993, Proc. Of the COSPAR Symp., Recent Results on X-ray and EUV Astronomy,
in press), an entirely new class of X-ray emitting objects unique for their
extremely soft X-ray spectra, the so-called super-soft sources, has been
classified and elucidated upon by ROSAT (Greiner et al. 1991, A&A, 246, L17).
The large volume of new information which ROSAT has already provided has
dramatically altered the CV landscape and the present "standard model" is
only standard in a transitory sense.

In an extended mission life the ROSAT HRI and its unique photometric
capabilities can focus on longer time scale projects which are not normally
carried out during the early phases of a space based mission. In particular
light curve mapping of eclipsing systems can further elucidate the extent of
the X-ray emitting regions in these systems and constrain the parameters of
the accretion disk corona which the previous ROSAT observations uncovered.
It is indeed ROSAT's unique soft X-ray response which makes this type of
program possible and insures a fruitful scientific outcome.

Neutron Stars: Neutron stars, by virtue of the energetics associated with
these dense compact objects, are arguably the canonical stellar system in
high energy astrophysics. Isolated neutron stars convert mechanical energy
of rotation into electromagnetic radiation via their large (B~10^12 G)
intrinsic surface magnetic fields; in many ways a cosmic dynamo. The advent
of soft X-ray detectors in orbit, initially Einstein but primarily ROSAT,
opened a whole new channel for the study of neutron stars and their structure
through the detection of the cooling radiation from the stellar surface, one
of the original goals of X-ray astronomy.

Prior to the launch of ROSAT there were only a handful of known X-ray emitting
isolated neutron stars. Presently, thanks to the unprecedented sensitivity of
the suite of ROSAT detectors, that number is now more than 20 with over a dozen
having pulsations either detected or confirmed during the mission. A brief
list of some of the highlights would read: the solution to the mystery of
Geminga (Halpern & Holt 1992, Nature, 357, 222) which was recognized to be a
radio quiet middle-aged isolated neutron star and had been an enigma since its
discovery in the gamma-ray domain over 20 years ago, the discovery of X-ray
pulsations from the Vela pulsar (Ogelman et al. 1993, Nature, 361, 163) as well
as the detection of a jet emanating from the pulsar which may be responsible
for carrying away the bulk of the spin down energy of the neutron star
(Markwardt & Ogelman 1995, Nature, 375, 40), the detection of the middle-aged
pulsars (age ~ 10^5-10^6 years) PSRs B0656+14 and B1055-52 (Finley et al. 1992,
ApJ, 394, L21; Ogelman & Finley 1993, ApJ, 413, L31) which, along with Geminga,
constitute the best candidate cooling neutron stars and are the crucial
experimental data necessary to begin to understand the equation of state of
matter at supra-nuclear densities, and the spectral details of the old neutron
stars (age ~ 10^6-10^7 years) PSRs B1929+10 and B0950+08 (Yancopoulos et al.
1994, ApJ, 429, 832; Manning & Willmore 1994, MNRAS, 266, 635) which are
indicative of emission from a magnetospherically heated polar cap. In short
the ROSAT mission has provided detailed data spanning some six orders of
magnitude in energetics which address very fundamental physics questions
related to neutron star dynamics and structure. This invaluable database
covers the entire X-ray life of a neutron star from youth through old age
and will continue to be a rich canvas upon which theorists can paint their
impressions.

The unique soft X-ray response and spatial capabilities of the ROSAT HRI
will allow the aforementioned database to be continually enriched during an
extended mission life. The excellent spatial response and photometric
capabilities of the HRI insure that with deep pointings more isolated neutron
stars will be detected. In fact, this assertion is supported by the recent
detection of a newly discovered middle-aged neutron star PSR J0538+2817 which
is a twin of the cooling candidate PSR B1055-52 (Finley 1996, private
communication). This discovery was possible as a result of the deeper and
more sensitive radio surveys which are being conducted and the neutron star
turns out to be one of the brighter X-ray emitting neutron stars. Thus, the
HRI will serve as a valuable tool as the pulsar catalog continues to rapidly
grow. The excellent spatial response of the HRI will also be useful in
revealing the morphology of neutron stars and the interactions of these high
velocity objects with their interstellar environs. A good recent example of
these type of studies are the detection of compact nebulae, similar to that
in which the Vela pulsar is embedded, associated with the young neutron stars
PSRs B1823-13 and B1706-44 (Finley 1996, Head meeting 1996, San Diego, CA).

Another important role of future HRI observations is discovering young neutron
stars in supernova remnants. These detections are important because it
facilitates a more complete understanding of the supernova phenomenon. An
example of this is what has been learned from a single such discovery, in
Puppis A (Petre et al. 1996, ApJL, in press). The oxygen abundances
determined from spectroscopy of the filaments requires the progenitor star
mass to be >325Mo; making this the most massive star known to leave a neutron
star remnant. In addition, the high proper motion of the neutron star
relative to the kinematic center (~1,000 km s^-1) and the fact that it is
moving diametrically opposite from the fast moving optical filaments whose
proper motion were used to determine the center, offers some of the strongest
evidence for an asymmetric supernova explosion. There are many bright
Galactic SNR for which HRI searches for neutron stars remain to be done.

X-Ray Binaries (XRBs): Due to their presence predominantly in the Galactic
plane the XRBs are typically heavily absorbed sources and as such have not
been the target of a large number of pointed observations. This is not to
say that ROSAT has not had its say in the matter. Timing studies of eclipsing
systems have determined the secular behavior of the orbital period in at least
one LMXB (Hertz et al. 1995, ApJ, 438, 385), a parameter crucial in sorting out
the possible evolutionary scenarios leading to the their formation, and
confirmed a persistent periodicity of ~ 3 hours which is not correlated to the
orbital period in a HMXB (Finley et al. 1992, A&A, 262, L25). The excellent
spatial resolution has also been useful in narrowing the search space for the
counterparts of XRBs. An excellent example was the recent HRI observation of
the bursting XRB GRO J1744-28 (Kouveliotou et al. 1996, IAUC 6286), which
suggested a source position different from that of all optical counterpart
candidates considered up to that time.

In an extended mission the HRI will prove a useful tool in the XRB area owing
to its excellent spatial resolution and photometric capabilities. Long time
scale projects such as orbital monitoring and eclipse mapping will be useful
in discriminating between various evolutionary scenarios for the formation of
XRBs. Follow-up observations of the transient sources which are now routinely
discovered by CGRO and RXTE will enable a cataloging of these objects with
sub-arcminute uncertainty, and thus a more constrained optical counterpart
searcy. Finally, deep HRI pointed observations of nearby galaxies will be
very important in understanding the distribution and evolution of XRBs.

Supernova (SN) explosions are the most energetic events in the interstellar
medium (ISM). They play a major role in the chemical and dynamical evolution
of the ISM in a galaxy. ROSAT HRI observations have provided essential
information on three evolutionary stages of SNe and their remnants in the ISM:
(1) the transition from SNe to supernova remnants (SNRs), (2) the structure
and evolution of SNRs, and (3) the large-scale diffuse X-ray emission powered
by multiple SNRs.

Supernovae and Their Young Remnants: Soon after a SN explosion, the expanding
ejecta encounter the circumstellar material that was shed by the SN prior to
the explosion. The interaction produces a fast outward moving shock as well
as a relatively slow backward moving shock into the stellar envelope. Both
shocks generate X-ray emission that carries information about the distribution
of circumstellar material and the development of a young SNR. Several <20 yr
old Type II SNe (with massive progenitors) have been detected in X-rays.
Massive stars are usually located in Population I environments that are likely
to contain SNRs and massive X-ray binaries. Therefore, it is crucial that
X-ray observations have adequate spatial resolution to resolve the SNe from
their background X-ray sources. The ROSAT HRI is up to the task. For
example, SN1986J in NGC 891 is resolved from a fainter source 28" away
(Houck & Bregman 1996, c). SN1988Z, recently detected by the HRI (Fabian &
Terlevich 1996, MNRAS, 280, L5), is especially noteworthy as the most distant
and most luminous SN detected in X-rays, with an X-ray luminosity exceeding
10^41 erg s^-1.

The ROSAT HRI has been used to monitor the evolution of young SNe. The most
exciting object is SN1987A in the
Large Magellanic Cloud (LMC). Its SN ejecta
are expected to encounter the progenitor's circumstellar shell within the next
few years. Because of its proximity,
SN1987A offers a unique opportunity for
us to watch the spatially resolved interaction between SN ejecta and the
circumstellar material. A few young SNe in more distant galaxies have also
been monitored. For example, SN1978K in NGC 1313 shows a surprisingly
constant X-ray light curve (Schlegel et al. 1996, ApJ, 456, 187). The
currently available X-ray observations of SNe are insufficient for a full
understanding of SNe evolution. Deep HRI observations are needed to detect
and to monitor a larger number of young SNe in nearby galaxies.

Supernova Remnants: In a fully developed SNR, X-ray emission is expected to
peak behind the shock fronts. However, if the ISM is clumpy and if a SNR has
engulfed many cloudlets, the SNR may show centrally peaked X-ray emission
(White & Long 1991, ApJ, 373, 543). The high spatial resolution of the HRI
provides an excellent means to study the physical condition and the shock
structure of SNRs in the Galaxy as well as in the Magellanic Clouds.

The temporal baseline of high resolution imaging is now long enough (six years
since the launch of ROSAT, eighteen since the launch of Einstein) to facilitate
measurements of the X-ray expansion of nearby, young remnants. This has now
been done for both
Tycho
(Hughes 1996 presented in the ASCA conference, Tokyo,
March 1996) and
Cas A
(Gotthelf & Keohane 1996, in preparation). Further
measurements of the expansion of these remnants, plus measurements of other
young remnants like Kepler and SN1006, can still be carried out using the HRI.
Deep images of the shells of nearby, older remnants such as
Puppis A may serve
as a baseline for expansion velocity measurements using AXAF.

The Cygnus Loop is the nearest SNR, hence can be studied at the highest spatial
resolution. The HRI mapping of the Cygnus Loop (a large observing program
requiring 10^6 s) has indeed produced a spectacular image allowing detailed
comparison between the X-ray morphology and optical images (Graham & Aschenbach
1996, ROSAT Workshop at the HEAD meeting, San Diego). It is possible to trace
the shock wave as a continuous surface and to closely examine its interaction
with interstellar clouds (Graham et al. 1995, ApJ, 444, 787). The analysis of
HRI mapping of the entire Cygnus Loop will yield the most complete information
about the structure and evolution of a SNR. Several apparently centrally
filled SNRs are being mapped by the HRI. A comparison between the X-ray and
Ha morphologies of their interiors will allow a critical comparison with White
& Long's (1991) model predictions.

The Magellanic Clouds, having little foreground absorption and being at small
distances, provide an excellent sample of SNRs that can be studied in detail
at multiple wavelengths. ROSAT HRI observations are needed to resolve these
SNRs; existing HRI observations have produced interesting results. The
SNR 0101-7226 in the Small Magellanic Cloud (SMC) does not seem to emit any
X-rays (Ye et al.1995, MNRAS, 275, 1218). The double shells of DEM L 316 are
thought to be a pair of colliding SNRs (Williams et al. 1996, BAAS, 28, 924).
The SNR N63A has an X-ray size much larger than its optical size; its
progenitor must have been a massive star whose wind blew a bubble within
which it exploded (Chu et al. 1996, submitted to AJ).

Diffuse X-Ray Emission: SNRs are responsible for heating the ~10^6 K
X-ray-emitting gas in a galaxy. This hot ionized phase of the ISM has been
the least well-known component because of the difficulties in making
observations of large-scale diffuse X-ray emission. The HRI survey of
the Magellanic Clouds (another large observing program, requiring 2.8x10^6 s)
has been extremely successful in not only detecting but also resolving the
diffuse X-ray emission (Snowden & Chu 1996; Chu et al. 1996, ROSAT Workshop
at the HEAD meeting, San Diego). Numerous SNRs, superbubbles, supergiant
shells, and large-scale diffuse emission are detected in the HRI survey.
Both optical emission line images and H I 21 cm line images of the Magellanic
Clouds are available (Smith et al. 1996, BAAS, 28, 900; Staveley-Smith & Kim
1996, BAAS, 28, 900). These multi-wavelength surveys of the Magellanic Clouds
will allow us to examine the global structure of an ISM for the first time.
The HRI survey is essential in providing information on the hot ionized medium.

The study of external galaxies provides considerable insight into a wide
variety of subjects ranging from discrete objects (e.g., accretion binaries,
SNRs, and supernovae), many of which have no Galactic counterpart, to diffuse
emission (e.g., galactic fountains and halos). Observations of nearby
galaxies such as the Magellanic Clouds and M31 can be used to obtain a more
complete catalog of certain classes of X-ray sources than even those of the
Milky Way, where foreground absorption in the Galactic disk can provide severe
constraints.

One of ROSAT's observational strengths has always been the study of
individual galaxies. While the surface brightness and spectral capabilities
of the PSPC, which facilitated searches for diffuse components and
determination of the nature of some discrete sources, are missed, the HRI is
a sensitive instrument for detecting and precisely locating discrete sources
and tracking their variability, and mapping the morphology of diffuse
emission. An example of these capabilities as well as some fascinating
results is the nearby galaxy NGC 1313, for which ~120 ks of HRI exposure has
been accumulated over the past several years (Petre & Schlegel 1996, in
preparation). A total of 11 discrete sources have been detected, all of those
observed by the PSPC, plus some additional ones in the confused center of the
galaxy. The light curves of the five brightest of these have been traced since
early in the mission; all have luminosities above 1x10^39 erg s^-1 (5 times
brighter than any Galactic binary), and all vary, except (strangely) the
unusual supernova SN1978k. The most variable, and fifth brightest, source
is coincident with the nucleus. In addition, the central region of the
galaxy contains diffuse emission tracing the spiral arms.

One of the very first HRI pointings provides another demonstration of the
power of the HRI. A 50 ks HRI pointing of the central region of
M31
detected 86 sources above a threshold of ~1.4x10^36 erg s^-1 (Primini et al.
1993, ApJ,
410, 615). Extrapolation of a flattening luminosity function below
2x10^37 erg s^-1 indicates that only 15-26 percent of the residual diffuse
emission observed within 1 kpc of the center can be attributed to unresolved
sources from this distribution. Comparison of the ROSAT data with data from
Einstein indicates that ~42 percent of the detected sources are variable.
Deep monitoring of the central region of M31 continues, and a large observing
program has been proposed to widen the spatial coverage within the galaxy.

In M32, a companion to M31, and the nearest elliptical galaxy, emission from
the central region of the galaxy is seen with a luminosity of a few
x10^38 erg s^-1 (Eskridge et al. 1996, ApJL, 463, L59). A brief HRI pointing
suggests the presence of at least three discrete sources; a proposed deep HRI
pointing can determine whether more are present.

Several key results involving HRI observations of diffuse emission have
recently been reported:

- The HRI observation of NGC 1399 reveals an anticorrelation between the radio
lobes and X-ray emission (Kim et al. 1995, ApJ, submitted). This suggests than
the hot, X-ray emitting gas provides a thermal pressure confinement for the
radio jets.

- The deep HRI observation of the colliding elliptical galaxies NGC 4782/4783
(3C278) shows the complexity of the X-ray emitting plasma in the system
(Colina & Borne 1995, ApJL, 454, L101). The image clearly details emission
from the galaxies, a bridge between the galaxies, tidal-like tails, and a sheet
of emission at their interaction boundary. In addition, models of the radio
jet indicate that it is likely being deflected by ram pressure from the hot
gas.

- The edge-on starburst galaxy NGC 2146 displays an extensive extended emission
resolved in both PSPC and HRI observations with a total luminosity is
~3x10^40 erg s^-1 (Armus et al. 1995, ApJ, 445, 666). In the HRI observation,
the emission is further divided into several bright knots on top of the general
diffuse emission. The extent of the X-ray emission is much greater than the
starburst activity observed at longer wavelengths. The X-ray emission along
the galactic minor axis is associated with Ha emission and dust filaments.

- The HRI image of the SABbc galaxy
NGC 4258 (M106) shows strong features
consistent with the twisted nuclear jets previously observed in radio
continuum and optical emission-line studies (Cecil et al. 1995, ApJ, 440,
181). The southeast jet is unresolved over its 2.5' (5 kpc) length which
contrasts to the more diffuse northwest jet. The data are consistent with
temperature of 0.3 keV with a luminosity of~1.6x10^40 erg s^-1. The strong
outflow of NGC 4258 provides a tight correlation between thermal X-rays and
the radio jet. The inferred temperature is consistent with that expected
from shocks with velocities observed in optical data.

One indication of the power of the HRI for the study of nearby galaxies is
that nearly half the large observing programs proposed in AO7 are to study
galaxies. Topics include the detailed mapping of the LMC, a complete census
of Local Group galaxies, detailed study of M31, and a survey of previously
unobserved nearby spirals. All these programs complement the science that
will be performed by AXAF as well as identify objects meriting deep imaging
and spectroscopy by AXAF.

Current HRI observations of active galactic nuclei (Seyfert galaxies, quasars,
radio galaxies, blazars) focus on three aspects: temporal variability on scales
from minutes to months, morphological studies, and detection of faint objects.

The HRI is an excellent photometer, and the X-ray variability of AGN has not
been well characterized on very short (less than one day) or very long (longer
than one week) timescales, nor has a comprehensive understanding of the
relationship between X-ray variability and that in other bands been
established. A number of ambitious observations of AGN, many in coordination
with one or more other orbiting observatories, seek to provide this
information. The most impressive such study was the monitoring of 3C390.3
every three days over a nine month interval (Leighly et al. 1996, in
Roentgenstrahlung from the Universe, eds. Zimmermann et al. p. 467). The
X-ray flux varies with much higher amplitude, and over much shorter time
scales, than does the optical emission. Monitoring of 4 low-redshift quasars
with the HRI over a period of about 1 month showed significant variability on
timescales of days -- both gradual dimming/brightening, and outburst. Quite
unexpected, if such variability proves to be common, this will place a crucial
constraint on models for X-ray emission in AGN. (Elvis et al. 1996, in
preparation).

HRI observations of nearby Seyfert galaxies show that kpc-scale extended
emission with typical luminosity 10^40-10^42 erg s^-1 is common. The extended
X-ray emission aligns with radio and/or narrow line region axes and has
spatial extent similar to that of the extended emission line gas. The
X-rays most likely arise as thermal emission from hot gas in pressure
equilibrium with optical line emitting gas and the synchrotron emitting
plasma responsible for the kpc scale radio jets and lobes (Wilson et al.
1996, in Roentgenstrahlung from the Universe, eds. Zimmermann et al. p. 529).

HRI observations are also revealing important new information about diffuse
X-ray emission from radio galaxies. Both for the low luminosity radio galaxy
NGC 1275 in the Perseus Cluster (Hasinger et al. MNRAS...) and for the high
luminosity (FR II) radio galaxy Cygnus A (Carilli et al. 1994, MNRAS, 270,
173), the expansion of the radio lobes evacuates a cavity in the hot (cluster)
gas surrounding the central galaxy. This allows the use of pressure balance
arguments to constrain models for the lobes. In Cygnus A, Harris et al.
(1994, Nature 367, 713) found for the first time a spatially resolved
structure for which the synchrotron self-Compton model provides the best
explanation of the data. This allows an estimate of the magnetic field
strength which does not rely on equipartition arguments, and implies that the
jets of Cygnus A (and presumably other radio sources) may be comprised of
e+/e- rather than p+/e- (relativistic) plasma.

A number of deep surveys have been conducted using the HRI, many of which are
follow up observations of PSPC deep survey pointings. The best known of these
is the Lockman Hole pointing, with a total exposure approaching 5x10^5 s, and
a point source flux sensitivity of 1x10^15 erg cm^-2 s^-1. The precise HRI
positions ease the search for optical counterparts, and split confused sources,
even at the detectability threshold. The HRI fluxes for the resolved sources
allow more precise estimates of the unresolved flux in the PSPC images. The
deep surveys are revealing that at low X-ray fluxes different source
populations contribute more strongly, especially the narrow line X-ray galaxies
(Hasinger 1996, in Roentgenstrahlung from the Universe, eds. Zimmermann et
al. p. 291).

An extraordinary number of projects remain to be performed on AGN using the
HRI, as chronicled in the contribution by M. Elvis to the San Diego workshop.
Among these are surveys of various classes of objects, from nearby galaxies
potentially housing micro-AGN, to the radio galaxies in the 3C catalog.
Several proposals for new deep survey fields were proposed in AO7, including
more than one for the Hubble deep field. An observation of this region would
dovetail with the high level of observational activity at all wavebands in
this field. The HRI is uniquely positioned to resolve the large number of
sources expected. The X-ray observation will help characterize cluster
evolution, and the contribution of starbursting galaxies and AGN to the X-ray
background.

The long exposures available during the HRI-only phase of the ROSAT mission
are enabling qualitatively new studies of clusters of galaxies to be made.
The higher background of the HRI compared to the PSPC (or the Einstein IPC
before it) typically meant that previously most observers sacrificed the
improved angular resolution for increased sensitivity to the low surface
brightnesses characteristic of clusters. Now, there are several situations
where the HRI is being used to advantage for cluster observations.

The first is to resolve fine details in low redshift objects. Huang and
Sarazin (1996, ApJ, 461, 622) have presented a high angular resolution study of
the nearby Hercules cluster. This cluster exhibits substructure on a rich
variety of scales, the smallest of which can only be studied with the HRI.
Two papers have presented observations of the interaction between X-ray
emitting cluster gas and the radio lobes of the brightest cluster galaxy,
which also happens to be a radio galaxy (Boehringer et al. 1993, MNRAS, 264,
L25 - NCG 1275; Carilli et al. 1994, MNRAS, 270, 173 - Cygnus A). Both papers
find that the X-ray emitting gas is displaced by the radio emitting material.
These observations provide strong support for the idea that jets of material
emanating from the nucleus power the radio galaxies. The effects of the jets
are clearly seen in the displaced X-ray gas.

Another situation is the observation of distant clusters where the PSPC does
not resolve the object. The availability of substantial amounts of HRI time
has vastly improved our knowledge of the X-ray emission properties of these
objects. Studies of distant clusters provide information on the rare objects
whose space densities are so small that nearby examples are unlikely to be
found. Edge et al. (1994, MNRAS, 270, L1) report HRI observations of Zw3146,
a cluster at z=0.291 which contains the largest cooling flow known,
1000 Mo y^-1. Schindler et al. (1996, A&A, submitted) describe the morphology
of the most luminous cluster known, RXJ1347.5-1145, which is at a redshift of
0.451 and has a bolometeric luminosity greater than 10^46 ergs s^-1. This cluster
may contain an even larger cooling flow, except its ASCA temperature is a hot
9.3 keV. Donahue (1996, ApJ, in preparation) is analyzing the morphology of
the most luminous cluster in the Einstein Medium Survey, MS0451.6-0305 at
z=0.539 with an ASCA temperature of 10.4 keV. This cluster, although
elongated, appears to be relaxed because there are no clumps containing
more than 1 percent of the total luminosity.

Another rare class of objects are clusters containing gravitationally lensed
arcs. Fortunately X-ray luminosity is correlated with a high likelihood of
the cluster being massive enough to image background galaxies. However, even
the high masses of very luminous objects only provide a weak lens so that
their focal lengths correspond to redshifts > 0.2 in most cases. A symbiotic
relationship has arisen between mass determinations of clusters by X-ray
methods and by gravitational lensing, both weak and strong. Each method has
its own systematic errors, so the combination of both yields more reliable
results. For example, one of the first applications of the weak lensing
technique was to MS 1224.7+2007 which yielded a mass corresponding to a
density parameter ~ 2 if this cluster were typical. HRI data (Henry et al.
1996, in preparation) show that there are two clusters superimposed on each
other but whose centers are displaced by 7'. The weak lensing result assumed
that the mass was zero at the edge of its field of view, 7' in this case, and
so is undoubtedly biased. After other apparent disagreements, more careful
analyses are showing that the two methods give the same results for the mass,
at least to better than factor of two (Allen et al. 1996, MNRAS, 279,
615 - PKS 0745-191; Squires et al. 1996, ApJ, 461, 572 - A 2218). Much work
remains to be done, because the detailed morphology of the mass deduced by
the two methods is not always in agreement, e.g., A2218 (Squires et al.,
1996; Kneib et al. 1995, A&A, 303, 27). Here even the morphology deduced from
the weak lensing disagrees with that from the strong. However, all is not
lost, sometimes the mass distributions coming from the two methods are in
excellent agreement, such as for A370.

ROSAT has provided a unique window on gravitational lensing systems. There
are a number of such systems in which the lensed object is observed in other
bands but the object serving as the lens is invisible. If the so-called
dark lens is a cluster of galaxies, its massive halo can in principle be
detected in X-rays. Such a detection has been made in the lens system
MG2016+112 in a 20 ks observation (Hattori et al. 1996, in X-Ray Imaging and
Spectroscopy of Cosmic Hot Plasmas). The lens is now identified as a cluster
of galaxies at z=1. If the mass of the lensing cluster can be estimated based
on the X-ray luminosity and extent, it can be used to estimate Ho (Grogin and
Narayan 1996, ApJ submitted). In the system 0957+561, X-ray counterparts of
the two images of the lensed quasar are detected, and while variability of
both components have been seen in both X-ray and other bands, the X-ray
variability does not track that seen at longer wavelengths (Chartas et al.
1995, ApJ, 445, 140). Chartas et al. suggest that at least one image is being
microlensed by stars or clumps of dark matter. In 0957+561, only an upper
limit can be placed on the luminosity of the lensing cluster. The success of
these observations demonstrates that an HRI campaign to observe more
gravitational lenses could yield valuable cosmological information; a number
of observations of lens systems were proposed in AO7.

The classical reason for observing distant objects is to study the evolution
of their properties. Henry et al. (1996, in preparation) have obtained HRI
observations of a complete sample of 10 Einstein Medium Survey clusters in
the range 0.3 - 0.4 (they also have ASCA observations of these same clusters).
Given the high percentage of substructure in low redshift clusters, ~40
percent, nearly every object in the distant sample should have such structure
if the density parameter were ~ 1. The morphologies of objects in the distant
sample contain the usual suspects: some with central peaks indicating cooling
flows with no substructure and some with evidence for mergers. These two
types are in about the same proportions as at low redshift (within the limited
statistics). There are two intriguing objects with peaked surface
brightnesses not centered on the overall cluster emission. Such morphology is
rare if nonexistent at low redshifts.

As the above examples show, studies of distant clusters with the ROSAT HRI
have reached a level comparable to that of early work on their nearby
counterparts. If ROSAT would continue to operate, X-ray images could be
obtained for a substantial fraction of all clusters known at redshifts beyond
0.3. Combined with ASCA temperatures, these data will permit investigation
into a range of problems of cosmological interest.